@Woland Hi there, thank you for your quick response.
It's a MacBook Pro 15" late 2016 which I thought was in its default resolution of 1680 x 1050 - when I go into "System Preferences" and "Displays" it's in default mode and this corresponds to 1680 x 1050 in my Screen Res app.
However, looking on the internet it says the native resolution for this Mac is 2880 x 1800, gosh have I been working at the wrong res all this time?! 😳 When I change to this resolution Isadora workspace looks fine - even if I then return to 1680 x 1050.
Many thanks for your help!